In the 1960s we used to use it as “Instant Decoke.”
This was in the days of dodgy fuel and sooted up engines.
However it did release gummed up piston rings, but not when running the engine.
First do a compression test to see just how bad it really was, it cannot fix a broken ring.
If it showed hints of promise, then the usual method was to squirt a teaspoon of it (or diesel) into each pot, turn the engine over by hand a couple of times and leave it to soak overnight. Next morning, turn it over slowly a few more times by hand and wait another day.
Be warned, cranking on the starter will throw redex out the plug holes and it goes absolutely everywhere.
Then came the fun bit.
First make sure the neighbours are away and the exhaust pipe faces out of the garage.
Then put on your gas mask, and fire up the engine for a minute or so, no more.
Then plugs out again and do the usual compression tests.
Sometimes it worked, but by no means always. However, it was always worth a try, for entertainment value if nothing else. And I cannot help smiling when remembering that French mechanic fixing the 2CV.